1. Technical Field
The invention relates to azaadamantane derivatives, and more particularly, crystalline monohydrate forms of azaadamantane derivatives, compositions comprising such compounds, methods of preventing or treating conditions and disorders using such compounds and compositions, and processes for preparing such compounds.
2. Description of Related Technology
Nicotinic acetylcholine receptors (nAChRs) are widely distributed throughout the central (CNS) and peripheral (PNS) nervous systems. Such receptors play an important role in regulating CNS function, particularly by modulating release of a wide range of neurotransmitters, including, but not necessarily limited to, acetylcholine, norepinephrine, dopamine, serotonin, and GABA. Consequently, nicotinic receptors mediate a very wide range of physiological effects, and have been targeted for therapeutic treatment of disorders relating to cognitive function, learning and memory, neurodegeneration, pain, inflammation, psychosis, sensory gating, mood, and emotion, among other conditions.
Many subtypes of the nAChR exist in the CNS and periphery. Each subtype has a different effect on regulating the overall physiological function. Typically, nAChRs are ion channels that are constructed from a pentameric assembly of subunit proteins. At least 12 subunit proteins, α2-α10 and β2-β4, have been identified in neuronal tissue. These subunits provide for a great variety of homomeric and heteromeric combinations that account for the diverse receptor subtypes. For example, the predominant receptor that is responsible for high affinity binding of nicotine in brain tissue has composition (α4)2(β2)3 (the α4β2 subtype), while another major population of receptors is comprised of homomeric (α7)5 (the α7 subtype) receptors.
Certain compounds, like the plant alkaloid nicotine, interact with all subtypes of the nAChRs, accounting for the profound physiological effects of this compound. While nicotine has been demonstrated to have many beneficial properties, not all of the effects mediated by nicotine are desirable. For example, nicotine exerts gastrointestinal and cardiovascular side effects that interfere at therapeutic doses, and its addictive nature and acute toxicity are well-known. Ligands that are selective for interaction with only certain subtypes of the nAChR offer potential for achieving beneficial therapeutic effects with an improved margin for safety.
The α7 and α4β2 nAChRs have been shown to play a significant role in enhancing cognitive function, including aspects of learning, memory and attention (Levin, E. D., J. Neurobiol. 53: 633-640, 2002). For example, α7 nAChRs have been linked to conditions and disorders related to attention deficit disorder, attention deficit hyperactivity disorder (ADHD), schizophrenia, Alzheimer's disease (AD), mild cognitive impairment, senile dementia, dementia associated with Lewy bodies, dementia associated with Down's syndrome, AIDS dementia, and Pick's disease, as well as inflammation. The α4β2 receptor subtype is implicated in attention, cognition, epilepsy, and pain control (Paterson and Norberg, Progress in Neurobiology 61 75-111, 2000) as well as smoking cessation or nicotine withdrawal syndrome.
The activity at both α7 and α4β2 nAChRs can be modified or regulated by the administration of subtype selective nAChR ligands. The ligands can exhibit antagonist, agonist, or partial agonist properties. Compounds that function as allosteric modulators are also known.
Although compounds that nonselectively demonstrate activity at a range of nicotinic receptor subtypes including the α4β2 and α7 nAChRs are known, it would be beneficial to provide compounds that interact selectively with α7-containing neuronal nAChRs, α4β2 nAChRs, or both α7 and α4β2 nAChRs compared to other subtypes.
In view of this need within the art, the inventors were involved with others, and developed the novel azaadamantane derivatives disclosed in co-pending publications. Specifically, U.S. Patent Application Publication No. 2008/0167336 (hereinafter referred to as the “'336 publication”) was directed to azaadamantane derivatives and the salt forms of the compound. The '336 publication indicates that the certain compounds disclosed therein may exist as salts of the compound of the invention. The salts of such compound include, but are not limited to, L-bitartrate anhydrate, L-bitartrate hydrate, dihydrogen phosphate anhydrate, dihydrogen phosphate hydrate, bisuccinate anhydrate, bisuccinate hydrate, hydrochloride quarterhydrate, hydrochloride sesquihydrate, dihydrogen citrate, and monohydrogen citrate. These compounds are considered important due to their selective interaction with α7-containing neuronal nAChRs, α4β2 nAChRs, or both α7 and α4β2 nAChRs compared to other compounds known in the art.